Optoelectronic directional couplers are used in many applications in modulating a lightwave or switching light signals in such as optical communication systems. A description of directional couplers is to be found in IEEE Journal of Quantum Electronics, vol. QE-12, No 7, July 1976, H. Kogelnik and R.V. Schmidt: .cent.Switched Directional Couplers with Alternating .DELTA..beta.". The directional couplers are intended for two coupled lightwave conductors, which are generally connected to their individual inputs and outputs of the coupler. The optical coupling between the lightwave conductors can be acted on with the aid of an electrical signal which is connected to electrodes at the coupled lightwave conductors. The directional couplers of the prior art have a transfer function according to which it is required that the electrical signal has a direct voltage level, an electrical bias, about which the modulating signal varies. In high frequency modulation, with modulation frequencies of about 5 GHz or higher, there are problems in keeping this level constant. A varying bias can cause an optical signal to be incompletely coupled and sent from both outputs of the directional coupler. A Mach-Zender type of bias-free modulator is illustrated in Appl. Phys. Lett 43 (11), Dec 1983, C.M. Gee, G.D. Thurmond and H.W. Yen: "17 GHz band-width electro-optic modulator". This modulator has wo wave conductors between which an incoming lightwave is divided, the lightwave being relatively phase-shifted in the respective wave conductors. The modulator has the disadvantage that it only has one input and one output, so that there lacks the availability of switching a signal between two outputs.